pt100 calculator

What is a PT100 sensor?

A PT100 is a platinum resistance temperature detector (RTD) with a nominal resistance of 100 Ω at 0°C. As temperature rises, resistance increases in a predictable, near-linear way. PT100 probes are widely used in industrial automation, HVAC, laboratories, food processing, and energy systems because they are stable, accurate, and repeatable.

Why engineers choose PT100

• Excellent accuracy: Better long-term stability than many thermocouples.
• Wide range: Typical model range is from about -200°C to +850°C.
• Good linearity: Easier signal conditioning and control loop tuning.
• Interchangeability: IEC 60751 gives standardized behavior.

How this PT100 calculator works

The calculator uses the standard Callendar–Van Dusen equations. You can calculate:

• Temperature → Resistance: Useful when simulating a sensor input for PLC testing.
• Resistance → Temperature: Useful when troubleshooting with a multimeter or calibrator.

Core equations

For temperature T ≥ 0°C:

R(T) = R₀ × (1 + A·T + B·T²)

For temperature T < 0°C:

R(T) = R₀ × (1 + A·T + B·T² + C·(T - 100)·T³)

Where R₀ is nominal resistance at 0°C (100 Ω for PT100), and A, B, C depend on the chosen curve standard.

Lead wire compensation (2-wire vs 3-wire vs 4-wire)

Lead resistance can cause meaningful temperature error if ignored, especially with long cable runs.

• 2-wire: Both lead wires add resistance and create the largest error.
• 3-wire: Most transmitters cancel much of lead error if leads are matched.
• 4-wire: Best method; essentially eliminates lead resistance influence.

This calculator includes a practical lead compensation estimate to help with field diagnostics.

Typical PT100 resistance checkpoints (IEC 60751, R₀ = 100 Ω)

• -200°C ≈ 18.52 Ω
• -100°C ≈ 60.26 Ω
• 0°C = 100.00 Ω
• 100°C ≈ 138.51 Ω
• 200°C ≈ 175.86 Ω
• 400°C ≈ 247.09 Ω
• 850°C ≈ 390.48 Ω

Practical calibration and troubleshooting tips

1) Verify the RTD type first

Always confirm whether your sensor is PT100, PT1000, or another RTD. A mismatch will produce large temperature interpretation errors.

2) Check transmitter input settings

Instruments often require selecting RTD curve type and wiring mode. If those settings do not match the actual installation, your displayed process value may drift or jump.

3) Validate with two points

A fast sanity check is to compare measured resistance against known checkpoints (for example around ambient and near a controlled hot point). Two-point verification catches many setup mistakes quickly.

FAQ

Is PT100 always exactly 100.000 Ω at 0°C?

Nominally yes, but real probes have tolerance classes (Class B, Class A, etc.). Manufacturing tolerance and installation conditions create small deviations.

Can I use this for PT1000?

Yes. Set R₀ = 1000. The same model applies, and the calculator scales accordingly.

Why does inverse conversion use iteration?

For temperatures below 0°C, the model includes a cubic-like correction term. Solving directly is not as simple, so numerical methods (bisection) provide robust and stable results.

Bottom line

If you work with process instrumentation, this PT100 calculator gives a fast, practical way to convert temperature and resistance, account for wiring effects, and troubleshoot sensor loops with confidence.